The invention relates to twin screw rotors for installation in displacement machines for compressible media, in particular pumps, which rotors are designed single-threaded with varying pitch and are intended to be in axis-parallel, opposed, outside engagement with wrapping angles of at least 720xc2x0, and to form in a housing an axial sequence of chambers without blow-hole connections, the end profile comprising a core arc of a circle, cycloidal hollow flank, an outer arc of a circle and a further flank.
The patent DE 87685 shows a machine called a screw wheel capsule mechanism, in which the screw rotors are designed with variable pitch. The machine can be used both as a motor and as a pump. In order to additionally increase the volume of the working chambers in the direction of the expanding medium, when operating as a motor, the rotors are alternatively constructed tapering.
Described in the patent DE 609405 is an air cooling machine with compressor and expander, which both have screw pairs with variable pitch and thread depth. The enveloping surfaces of the rotors are designed tapering.
Both of the aforementioned machines have the drawback that they require tapering cylinders, whereby the rotors can be put in and taken out on one side only. This increases the expenditure (of time and effort) during assembly and disassembly of the machines, which is very disadvantageous in particular during maintenance and repair work.
The patent application EP 0 697 523 concerns a screw-type displacement machine in which the rotors meshing with one another have unequal screw profiles, designated as xe2x80x9cmalexe2x80x9d and xe2x80x9cfemalexe2x80x9d, so-called S.R.M. profiles with continuous change of the pitch. The end profile is thereby varied in such a way that the angle of the tooth tip, or respectively the length of outer arc is a monotonically increasing function dependent upon the wrapping angle. Such profiles have the drawback that a good partitioning of the axial sequence of working cells is not possible owing to the remaining blow hole. The vacuum losses caused by the blow hole result in losses in degree of efficiency, so that with such a machine no good inner compression is possible at least at low and medium rotational speeds.
The published patent application DE 19530662 discloses a screw suction pump with outer combing screw elements, in which the pitch of the screw elements continuously decreases from their inlet end to their outlet end to cause the compression of the gas to be released. The shape of the teeth of the screw rotor has an epitrochoidal and/or Archimedean curve. This machine has the disadvantage that the achievable inner compression rate with the shown geometric proportions is mediocre. Moreover the lacking end profile variation sets the already not good compression ratio, and leads to an increased leak rate owing to the reduction in the depth of the gap between screw outer diameter and housing toward the screw end.
The published patent application DE 4445958 describes a screw compressor with opposite-running, rotating outer combing screw elements. The threading helices of the screw elements become continuously smaller from one axial end to the second axial end removed therefrom. Proposed as a profile is a rectangular or trapezoidal profile. A drawback of these kinds of geometry for the profile is that they work sufficiently loss-free only if the thread depth is minimal with respect to the diameter, as is explained in the said publication. Such a machine therefore has a large constructional volume and a great weight. It is a further drawback of such profile geometry that extremely high changes in pitch are necessary if a satisfactory inner compression rate is supposed to be achieved. As with the mentioned DE 19530662, the lacking end profile variation here too fixes this shortcoming, and this leads to a high leak rate owing to the reduction of the depth of the gap between screw outer diameter and housing toward the screw end.
Further publications, such as, for example, SE 85331, DE 2434782 and DE 2434784, concern inner-axial screw machines with non-constant pitch of the screws or varying end profiles. These machines all have the drawback that the construction cost is high, and that in each case dynamic seals are also required suction-side.
Furthermore there are some publications, for example DE 2934065, DE 2944714, DE 3332707 and AU 261792, which describe two-shaft compressors with screw-like rotors. There the rotors, and in some cases the housing too, are composed of profile disks of differing thickness and/or contour, disposed axially behind one another, and thus achieve inner compression. All machines with screw-like rotors have the drawback that their degree of efficiency is decreased compared to that of machines with screw-shaped rotors because detrimental spaces and whirl zones arise through the stepped construction. Furthermore problems with respect to shape consistency are to be expected with screw-like rotors since they heat up in operation.
Starting with this state of the art, the invention has as its object to propose a twin screw rotor which does not have the above-mentioned drawbacks.
These objects are attained, according to the invention, in that the pitch is not monotone and is defined as a variable dependent upon the wrapping angle, in that the pitch in a first section increases from the suction-side screw end, and reaches a maximal value after approximately one turn, in that the pitch in a second section, adjacent to the first section, decreases and reaches a minimal value at approximately one turn before the delivery side screw end, and in that the pitch in a third section, adjacent to the second section, remains substantially constant.
Special embodiments of the invention are described in the dependent claims.